In a trend for downsizing electronic devices, there is increasing demand for printed circuit boards, serving as main elements of the devices. Currently, such printed circuit boards have high element integration density and a number of layers. Thus, when there is a large difference in coefficient of thermal expansion (CTE) between the semiconductor-sealing material and the substrate board, the circuit on the board may be broken. In order to solve this problem, the CTE of the semiconductor-sealing material is reduced through a technique in which amorphous silica—an inorganic substance having small CTE—is added as a filler to a heat-resistant resin (e.g., an epoxy resin). However, amorphous silica has a positive CTE, although its value is small. Thus, even when amorphous silica is added to an epoxy resin or the like, the CTE of a sealing material made of the thus-prepared resin material cannot be sufficiently reduced.
Meanwhile, β-eucryptite (β-LiAlSiO4) is a metal oxide having a negative CTE. However, when β-eucryptite is used as a filler for reducing the CTE of a semiconductor-sealing material, the particle size of β-eucryptite must be reduced to a maximum extent for enhancing the filling density in a resin material.
Patent Document 1 discloses an eucryptite-based filler for use in an insulating composite material. In Patent Document 1, eucryptite is synthesized from LiO2, SiO2, and Al2O3 as starting materials, and a thermal treatment at 1,000 to 1,400° C. is preferably performed for synthesizing single-phase eucryptite.
Patent Document 2 discloses a technique including mixing aqueous solutions of lithium chloride, aluminum chloride, and sodium silicate, subsequently forming nano-scale seed particles via precipitation or deposition, removing sodium ions as an impurity through a specific method, and heating the product at 700 to 1,300° C.